Pixel with global shutter

ABSTRACT

A pixel includes a photosensitive circuit, a sense node, a first transistor and a first capacitor. A first electrode of the first capacitor is connected to a control terminal of the first transistor. A second electrode of the first capacitor is to a node of application of a first control signal.

TECHNICAL FIELD

The present disclosure generally concerns image sensors, and moreparticularly the pixels of such image sensors.

BACKGROUND

Image sensors are known in the art which are global shutter sensorsglobal shutter pixels. These pixels typically include a large number oftransistors. There is a need in the art to provide a pixel supporting aglobal shutter operation which comprises a decreased number oftransistors as compared with known pixels, particularly as compared withglobal shutter pixels.

SUMMARY

In an embodiment, a pixel comprises: a photosensitive area; a sensenode; a first transistor; and a first capacitor having a first electrodeconnected to a control terminal of the first transistor and having asecond electrode connected to a node of application of a first controlsignal.

The first transistor is connected between a node of application of apower supply potential and an output node of the pixel, the controlterminal of the first transistor being coupled to an inner node of thepixel.

The pixel further comprises a second transistor having a controlterminal connected to the sense node, having a first conduction terminalconnected to the inner node, and having a second conduction terminalconnected to a first node of application of the power supply potentialor of a second control signal.

The first node is a node of application of the second control signal andno transistor configured to deliver a bias current to the secondtransistor is connected to the inner node.

The pixel further comprises a third transistor having a first conductionterminal connected to the sense node and having a control terminalconnected to a node of application of a third control signal. A secondconduction terminal of the third transistor is connected to a node ofapplication of the power supply potential. Alternatively, a secondconduction terminal of the third transistor is connected to a node ofapplication of a fourth control signal.

The first transistor is an output transistor, while the secondtransistor is a readout transistor and the third transistor is a resettransistor. So, the first control signal is an output control signal,while the second control signal is a signal for controlling the secondtransistor, the third control signal is a reset control signal and thefourth control signal is a second reset control signal.

The pixel further comprises: a storage selection transistor connectedbetween the first electrode of the first capacitor and the inner node; asecond output transistor having a first conduction terminal connected toa node of application of the power supply potential, having a secondconduction terminal connected to a second output node and having acontrol terminal coupled to the inner node; and a second capacitorhaving a first electrode connected to the control terminal of the secondoutput transistor and having a second electrode connected to a node ofapplication of a second output control signal.

The pixel further comprises: a second capacitor having a first electrodecoupled to the inner node and having a second electrode connected to anode of application of a second output control signal; and a storageselection transistor connected between the first electrode of the firstcapacitor and a first electrode of the second capacitor.

The first electrode of the second capacitor is connected to the innernode, or the pixel comprises a second storage selection transistorconnected between the first electrode of the second capacitor and theinner node.

The pixel further comprises a transfer transistor connected between thephotosensitive area and the sense node.

In an embodiment, an image sensor comprises an array of pixels such asdefined hereabove.

In an embodiment, a method for controlling a pixel such as definedhereabove comprises: turning off the first transistor by setting thefirst control signal to a first potential and, to read out an outputsignal of the pixel, turning on the first transistor by setting thefirst control signal to a second potential.

In an embodiment, a pixel comprises: a photosensitive area; a sensenode; a first transistor having a control terminal connected to thesense node and having a first conduction terminal connected to a node ofapplication of a first control signal.

A second conduction terminal of the first transistor is connected to aninner node, with no transistor configured to deliver a bias current tothe first transistor being connected to the inner node.

The pixel further comprises a second transistor having a controlterminal coupled to the inner node, having a first conduction terminalconnected to a node of application of a power supply potential, andhaving a second conduction terminal coupled to an output node.

The pixel comprises a first capacitor having a first electrode connectedto the control terminal of the second transistor.

The first transistor is a readout transistor and the second transistoris an output transistor. The first control signal is a signal forcontrolling the first transistor.

The pixel further comprises a reset transistor having a first conductionterminal connected to the sense node, having a control terminalconnected to a node of application of a reset control signal, and havinga second conduction terminal connected to a first node of application ofthe power supply potential or of a second reset control signal.

The pixel further comprises a second capacitor and a storage selectiontransistor connected between the first electrode of the first capacitorand a first electrode of the second capacitor, the first electrode ofthe second capacitor being coupled to the inner node, or the pixelcomprises: a storage selection transistor connected between the firstelectrode of the first capacitor and the inner node; a second outputtransistor having a first conduction terminal connected to a node ofapplication of the power supply potential, having a second conductionterminal coupled to a second output node of the pixel, and having acontrol terminal coupled to the inner node; and a second capacitorhaving a first electrode connected to the control terminal of the secondoutput transistor.

A second electrode of each capacitor is connected to a node ofapplication of a reference potential, with the pixel further comprisinga row selection transistor connected between the second conductionterminal of the output transistor and the output node.

Alternatively, a second electrode of each capacitor is connected to anode of application of a readout control signal, with the secondconduction terminal of the output transistor being connected to theoutput node.

The second conduction terminal of the second output transistor isconnected to the second output node, or the pixel comprises a second rowselection transistor connected between the second conduction terminal ofthe second output transistor and the second additional output node.

The first node is a node of application of the power supply potentialand the pixel further comprises a second row selection transistorconnecting the first electrode of the second capacitor to the innernode, or the first node is a node of application of the second resetcontrol signal and the first electrode of the second capacitor isconnected to the inner node.

The pixel further comprises a transfer transistor connected between thephotosensitive area and the sense node.

Another embodiment provides an image sensor comprising an array ofpixels such as defined hereabove.

In another embodiment, a method of controlling a pixel such as definedhereabove, comprises, for each storage representative of a potentiallevel of the sense node, the first control signal applied to the nodehaving the conduction terminal of the first transistor connected theretois a potential ramp.

In another embodiment, a pixel comprises: a photosensitive area; a sensenode; a first transistor having a control terminal connected to thesense node; and a second transistor having a conduction terminalconnected to the sense node, having a control terminal connected to anode of application of a first control signal, and having its otherconduction terminal connected to a node of application of a secondcontrol signal.

The first conduction terminal of the first transistor is connected to aninner node and a second conduction terminal of the first transistor isconnected to a node of application of a power supply potential or to anode of application of a third control signal.

The second terminal of the readout transistor is connected to the nodeof application of the third control signal and no transistor configuredto deliver a bias current to the first transistor is connected to theinner node.

The pixel further comprises a third transistor having a first conductionterminal connected to a node of application of the power supplypotential, having a second conduction terminal coupled to an outputnode, and having a control terminal coupled to the inner node. The pixelfurther comprises a first capacitor having a first electrode connectedto the control terminal of the third output transistor; and a fourthtransistor connected between the first electrode of the capacitor andthe inner node.

The pixel further comprises a second capacitor having a first electrodeconnected to the inner node.

The first transistor is a readout transistor, the second transistor is areset transistor, the third transistor is an output transistor and thefourth transistor is a storage selection transistor. So, the firstcontrol signal is a reset control signal, the second control signal is asecond reset control signal and the third control signal is a signal forcontrolling the first transistor.

A second electrode of each capacitor is connected to a node ofapplication of an output control signal, the second conduction terminalof the output transistor being connected to the output node.

Alternatively, a second electrode of each capacitor is connected to anode of application of a reference potential, the pixel furthercomprising a row selection transistor connected between the secondconduction terminal of the output transistor and the output node.

The pixel further comprises a second output transistor having a firstconduction terminal connected to a node of application of the powersupply potential, having a second conduction terminal coupled to asecond output node of the pixel, and having a control terminal connectedto the inner node.

The second conduction terminal of the second output transistor isconnected to the second output node, or the pixel further comprises asecond row selection transistor connected between the second conductionterminal of the second output transistor and the second output node.

The pixel further comprises a transfer transistor connected between thephotosensitive area and the sense node.

In an embodiment, an image sensor comprises an array of pixels such asdefined hereabove.

In an embodiment, a method for controlling a pixel such as definedhereabove, wherein a storage representative of a potential level of thesense node, comprises: turning on the second transistor by switching thefirst control signal and setting the second control signal to a valuecapable of turning off the first readout transistor.

In an embodiment, a pixel comprises: a photosensitive area; a sensenode; a first transistor having a control terminal connected to thesense node and having a first conduction terminal connected to an innernode; a second transistor having a first conduction terminal connectedto the inner node; a coupling capacitor and a first storage capacitorseries-connected between the control terminal of the readout transistorand a node of application of a reference potential.

A second conduction terminal of the second transistor is capable ofbeing selectively coupled to a node of application of the referencepotential or to a node for supplying an output signal of the pixel.

A capacitance value of the coupling capacitor is at least ten timessmaller than that of the first storage capacitor.

A second conduction terminal of the first transistor is connected to anode of application of a power supply potential.

The pixel further comprises a third transistor having a first conductionterminal connected to the junction node of the first storage capacitorand of the coupling capacitor, and having a second conduction terminalcoupled to the inner node.

The pixel further comprises a second storage capacitor connected betweenthe second conduction terminal of the third transistor and a node ofapplication of the reference potential. The pixel further comprises afourth transistor connected between the second conduction terminal ofthe third transistor and the inner node.

A control terminal of the second transistor is connected to a node ofapplication of a first control signal; and a control terminal of thethird transistor is connected to a node of application of a secondcontrol signal.

A control terminal of the fourth transistor is connected to a node ofapplication of a third control signal.

A fifth transistor is connected between the photosensitive area and thesense node, a control terminal of the fifth transistor being connectedto a node of application of a fourth control signal.

The first transistor is a readout transistor, the second transistor isan output and biasing transistor, the third transistor is a firstselection transistor, the fourth transistor is a second selectiontransistor and the fifth transistor is a transfer transistor.

In an embodiment, an image sensor comprises an array of pixels such asdefined hereabove.

According to the fourth aspect, another embodiment provides a method ofcontrolling a pixel such as defined according to the fourth aspect or ofa pixel of an image sensor such as defined according to the fourthaspect, the method comprising: storing a voltage across the firststorage capacitor, said voltage being representative of a potentiallevel of the sense node; and reading out the potential level imposed tothe first inner node by the coupling capacitor and the first transistor.

To store said voltage, the third transistor is switched from the onstate to the off state.

To impose the potential level of the inner node by means of the couplingcapacitor and of the first transistor, the fourth transistor ismaintained off.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages will be discussed indetail in the following non-limiting description of specific embodimentsin connection with the accompanying drawings, wherein:

FIG. 1 shows an embodiment of a pixel circuit according to a firstaspect of the present disclosure;

FIG. 2 is a flowchart illustrating an embodiment of a method ofcontrolling the pixel of FIG. 1;

FIG. 3 shows an alternative embodiment of the pixel circuit of FIG. 1;

FIG. 4 shows an embodiment of a pixel circuit according to a secondaspect of the present disclosure;

FIG. 5 is a flowchart illustrating an embodiment of a method ofcontrolling the pixel of FIG. 4;

FIG. 6 shows an alternative embodiment of the pixel circuit of FIG. 4;

FIG. 7 shows an embodiment of a pixel circuit according to a thirdaspect of the present disclosure;

FIG. 8 is a flowchart illustrating an embodiment of a method ofcontrolling the pixel of FIG. 7;

FIG. 9 shows an alternative embodiment of the pixel circuit of FIG. 7;

FIG. 10 shows an embodiment of a pixel circuit corresponding to thecombination of the embodiments of FIGS. 1, 4 and 7;

FIG. 11 shows an alternative embodiment of the pixel of FIG. 10;

FIG. 12 shows an embodiment of a pixel circuit according to a fourthaspect of the present disclosure; and

FIG. 13 is a flowchart illustrating an embodiment of a method ofcontrolling the pixel of FIG. 12.

DETAILED DESCRIPTION

The same elements have been designated with the same reference numeralsin the different drawings. In particular, the structural and/orfunctional elements common to the different embodiments may bedesignated with the same reference numerals and may have identicalstructural, dimensional, and material properties.

For clarity, only those steps and elements which are useful to theunderstanding of the described embodiments have been shown and aredetailed. In particular, the details of the calculations enabling todetermine the quantity of light received by a photosensitive orphoto-conversion area of a pixel from two signals representative, at twodifferent times, of the potential of a sense node of the pixel (doublesampling) have not been described, it being within the abilities ofthose skilled in the art to implement such calculations.

Throughout the present disclosure, the term “connected” is used todesignate a direct electrical connection between circuit elements withno intermediate elements other than conductors, whereas the term“coupled” is used to designate an electrical connection between circuitelements that may be direct, or may be via one or more intermediateelements.

In the following description, when reference is made to terms qualifyingabsolute positions, such as terms “front”, “back”, “top”, “bottom”,“left”, “right”, etc., or relative positions, such as terms “above”,“under”, “upper”, “lower”, etc., or to terms qualifying directions, suchas terms “horizontal”, “vertical”, etc., unless otherwise specified, itis referred to the orientation of the drawings.

In the following description, when reference is made to a bias, powersupply, or reference potential applied to a node, unless otherwiseindicated, this means that the potential has a value provided to beconstant during the pixel operation, although, in practice, this valuemay be only approximately constant. Further, when reference is made to acontrol signal, for example, applied to a node, unless otherwiseindicated, this means an analog signal having a continuously-varyingvalue or a digital signal alternating between first and second constantvalues, where the first and second values may in practice beapproximately constant only and may be different for two differentdigital signals. When reference is made to a control signal with nofurther precision, this means a digital control signal. Further, forsimplification, on-state voltage drops in the transistors switching inall or nothing are neglected in the discussion of the operation.

In the following description, unless otherwise indicated, a potential isreferenced to a reference potential, preferably ground GND.

In the following description, unless otherwise indicated, the source andthe drain of the transistor are called conduction terminals of a MOStransistor and the transistor gate is called control terminal.

In the following description, unless otherwise indicated, when a MOStransistor is connected between two nodes, terminals, and/or elements,this means that the two nodes, terminals, and/or elements are connectedto the respective conduction terminals (source and drain) of thetransistor.

The terms “approximately”, “substantially”, and “in the order of” areused herein to designate a tolerance of plus or minus 10%, preferably ofplus or minus 5%, of the value in question.

FIG. 1 shows a first embodiment of a circuit of a pixel 1 according to afirst aspect of the present disclosure.

Pixel 1 comprises a photosensitive area 100, that is, an area ofconversion of photons into electric charges. Photosensitive area 100 is,for example, a photodiode having a first electrode, for example, itsanode, coupled, preferably connected, to a node 102 of application of areference potential, preferably ground GND. A transfer MOS transistor106, for example, an N-channel transistor, connects the other electrodeof photodiode 100, in this example, its cathode, to a node 104. In thisexample, the source of transistor 106 is connected to the cathode ofphotodiode 100. The gate of transistor 106 is connected to a node 108 ofapplication of a control potential or digital signal TG. Node 104 is thepixel sense node and is intended to receive photogenerated chargestransferred from photodiode 100.

Pixel 1 also comprises a MOS transistor 110 for resetting node 104, forexample, an N-channel transistor. The gate of transistor 110 isconnected to a node 112 of application of a reset control potential ordigital signal RST. Transistor 110 is connected between sense node 104and a node 114, the source of transistor 110 being, in this example,connected to node 104. In this embodiment, node 114 is a node ofapplication of a bias potential, here a power supply potential VDD, forexample, positive.

Pixel 1 further comprises a MOS transistor 116 for reading from node104, for example, an N-channel MOS transistor 116. Transistor 116 hasits gate connected to node 104. Transistor 116 is connected between aninner (intermediate) node 118 of the pixel and a node 120, the source ofthe transistor being, in this example, connected to node 118. In thisembodiment, node 120 is a node of application of power supply potentialVDD.

Pixel 1 further comprises a MOS transistor 122 for biasing transistor116, for example, an N-channel transistor. The gate of transistor 122 isconnected to a node 124. In this embodiment, node 124 is a node ofapplication of a bias potential VBIAS having its value, for example, 0.6V, selected so that a bias current flows through transistor 116.Transistor 122 is connected between node 118 and a node 126, the sourceof transistor 122 being, in this example, connected to node 126. In thisembodiment, node 126 is a node of application of reference potentialGND.

In the embodiment of FIG. 1, pixel 1 comprises two storage selection MOStransistors 1281 and 1282, for example, N-channel transistors. The gatesof transistors 1281 and 1282 are connected to respective nodes 1301 and1302 of application of storage selection control potentials or digitalsignals, respectively S1 and S2. Transistors 1281 and 1282 are connectedbetween inner node 118 and gates, respectively 1371 and 1372, of outputMOS transistors, respectively 1321 and 1322, of the pixel. In otherwords, transistors 1281 and 1282 are series-connected between nodes 1371and 1372, their junction node being connected to node 118. In thisexample, the drains of transistors 1281 and 1282 are connected to node118. Each output transistor 1321 and 1322, for example, an N-channeltransistor, is (directly) connected between a node 1341, respectively1342, of application of power supply voltage VDD, and an output node1361, respectively 1362, of the pixel. In this example, the sources oftransistors 1321 and 1322 are (directly) connected to respective outputnodes 1361 and 1362. A capacitor C1 is (directly) connected between gate1371 of transistor 1321 and a node 1381 of application of a rowselection control potential or digital signal, or output control signal,READ1. A capacitor C2 is (directly) connected between gate 1372 oftransistor 1322 and a node 1382 of application of a row selectioncontrol potential or digital signal, or output control signal, READ2.Nodes 1381 and 1382 may be connected, with control signals READ1 andREAD2 then corresponding to a same signal. In this embodiment, signalsREAD1 and READ2 are signals having one of their states which isnegative, signals READ1 and READ2 switching between ground GND and anegative potential.

Transistors 1301 and 1321 and capacitor C1 define a first output circuitof the pixel, connected between nodes 118 and 1361 of the pixel.Transistors 1302 and 1322 and capacitor C2 define a second outputcircuit of the pixel, connected between nodes 118 and 1362 of the pixel.

As a specific embodiment, each capacitor C1 and C2 is formed by means ofa conductive wall covered with an insulator, or insulated conductivewall, filing a ring-shaped trench surrounding a doped portion of asemiconductor substrate. An electrode of the capacitor then correspondsto the conductor of the wall, the other electrode corresponding to thedoped semiconductor portion surrounded by the trench. A trench filledwith an insulated conductive wall is currently known to those skilled inthe art as a capacitive deep trench isolation or CDTI trench.

FIG. 2 is a flowchart illustrating an embodiment of a method ofcontrolling the pixel of FIG. 1.

At a step 20 (Reset SN), sense node 104 is reset. To achieve this,control signal RST is set to its high value, for example, potential VDD,to turn on transistor 110. Transistor 110 is then set to the off state,by switching signal RST to its low value, for example, ground GND. Thepotential of node 104 is then substantially equal to potential VDD ofnode 114.

Step 20 is implemented while an integration phase, during which chargesare photogenerated and stored in photodiode 100, is going on.Preferably, step 20 is implemented towards the end of the integrationperiod. In this example, the photogenerated charges which are stored inphotodiode 100 are electrons.

Further, during step 20, control signal TG is maintained at its lowvalue, for example, ground GND, such that transistor 106 is off. Controlsignals READ1 and READ2 are preferably maintained at their high value,here, ground GND. Further, control signals S1 and S2 are preferablymaintained at their high value, for example, voltage VDD, such that thecorresponding storage selection transistors (1281 and 1282 in thisembodiment) are on. Due to the fact that transistor 116 is configured asa source follower and is biased by transistor 122, the potential of node118, and thus the voltage across capacitors C1 and C2, depends on thepotential of node 104.

At a step 21 (Sample C1) following step 20, information representativeof the potential of node 104 is stored in capacitor C1. For thispurpose, signal S1 is switched to its low value, for example, groundGND, causing the turning off of the corresponding storage selectiontransistor (1281 in this embodiment).

At a step 22 (Transfer) following step 21, the photogenerated chargesstored in photodiode 100 during the integration phase are transferred tonode 104. For this purpose, control signal TG is switched to its highvalue, for example potential VDD, causing the switching of transistor106 to the on state. The charge transfer causes a modification of thepotential of node 104, the potential of node 104 then beingrepresentative of the quantity of transferred charges, and thus of thequantity of light received by photodiode 100 during the integrationperiod which is about to end. The potential variation of node 104 causesa corresponding variation of the potential of node 118, and thus of thevoltage across capacitor C2. At the end of step 22, transistor 106 turnsoff. Such a switching of transistor 106 from the on state to the offstate marks the end of the integration period and, for example, thebeginning of the next integration period.

At a step 23 (Sample C2) following step 22, information representativeof the potential of node 104 is stored in capacitor C2. To achieve this,signal S2 is switched to its low value, for example, ground GND, causingthe turning off of the corresponding storage selection transistor (1282in this embodiment).

At the end of step 23, controls signals READ1 and READ2 are switched totheir low (negative values), which causes a corresponding offset of thevoltage of nodes 1371 and 1372. The negative low values of potentialsREAD1 and READ2 are selected so that output transistors 1321 and 1322 ofthe pixel are off. As an example, particularly when capacitors C1 and C2are formed from trenches CDTI, the low values of potentials READ1 andREAD2 are equal to approximately −1.2 V.

At a step 24 (READ C1, C2) following step 23, the voltages acrosscapacitors C1 and C2 are read out, these voltages being representativeof the potentials of node 104, respectively after reset step 20 andafter step 22 of transfer of the photogenerated charges. Step 24corresponds to a step of reading from a pixel.

To achieve this, signals READ1 and READ2 are switched from their lowvalues to their high values, which causes a corresponding offset of thepotential of nodes 1371 and 1372. The high values of potentials READ1and READ2, here, ground GND, are selected so that output transistors1321 and 1322 turn on and remain so during the entire reading from thepixel. Due to the fact that transistors 1321 and 1322 are configured asa source follower and are biased by a readout circuit (not shown)connected to nodes 1361 and 1362, the potential of nodes 1361 and 1362depends on the voltage of the respective nodes 1371 and 1372. Thepotentials of nodes 1361 and 1362, corresponding to output signals ofthe pixel, are read out by the readout circuit. The quantity of lightreceived by photodiode 100 during the previous integration phase canthen be deduced from these output signals, particularly from thedifference between these signals.

Steps 20 to 24 are repeated for each integration phase.

Pixel 1 may be used to form an image sensor comprising an array ofpixels 1 organized in rows and in columns, a control circuit or rowdecoder, and a readout circuit or column decoder. The row decoder ispreferably common to all the pixels in the array. The row decodersupplies bias potential VBIAS and control signals TG, RST, READ1, READ2,S1, and S2, simultaneously to all the pixels of a same row. Further, ineach pixel column, output nodes 1361 and 1362 are common to all thepixels in the column, each node being coupled, preferably connected, toan input of the readout circuit.

In such a sensor, steps 20, 21, 22, and 23 are preferably implementedsimultaneously for all the pixels in the array. As a result, eachintegration phase starts and ends simultaneously for all the sensorpixels, which corresponds to an operation of global shutter type.However, the pixel rows are successively read from or selected while anintegration phase is going on, by implementing step 24, row after row,simultaneously for all the pixels in the selected row. The provision ofsignals READ1 and READ2 having their high and low values selected sothat the output transistors of the pixel are respective on and offenables to ensure that, for a given column, an output signal read out bythe readout circuit only depends on the pixel having had its rowselected.

To successively read from the rows of pixels 1 of the sensor, it couldhave been devised to add a row selection transistor controlled by asignal READ1, between transistor 1321 and node 1361, and a row selectiontransistor controlled by a signal READ2, between transistor 1322 andnode 1362, and to apply a bias potential rather than a control signal tonodes 1381 and 1382. However, this would have resulted in an increase inthe number of transistors of pixel 1.

FIG. 3 shows an alternative embodiment of the pixel circuit of FIG. 1.

Pixel 1′ of FIG. 3 differs from that of FIG. 1 in that it comprises asingle output node 136 and a single output circuit, connected betweeninner node 118 and node 136. In the shown example, the single outputcircuit comprises the two capacitors C1 and C2.

More particularly, in FIG. 3, the output circuit of pixel 1′ comprisestwo storage selection MOS transistors 1401 and 1402, for example, withan N channel, series-connected between the gate of an output MOStransistor 132, for example, with an N channel, and node 118. In thisexample, transistor 1402 is connected to node 118, transistor 1401 beingconnected to the gate of transistor 132. The gates of transistors 1401and 1402 are connected to respective nodes 1421 and 1422 of applicationof the voltages of digital control signals, respectively S1 and S2.Capacitor C1 is (directly) connected between a node 1441 of applicationof signal READ1 and the gate of transistor 132, corresponding in thisexample to the source of transistor 1401. Capacitor C2 is (directly)connected between a node 1442 of application of signal READ2 andjunction node 1452 of transistors 1401 and 1402, here corresponding tothe source of transistor 1401 and to the drain of transistor 1402.Output transistor 132 is (directly) connected between output node 136and a node 134 of application of a power supply potential VDD, thesource of transistor 132 being, in this example, connected to node 136.

As with the pixel of FIG. 1, nodes 1441 and 1442 may be connected andthus control signals READ1 and READ2 then correspond to a same signal.

The operation previously described in relation with pixel 1 of FIG. 1can be transposed to pixel 1′ of FIG. 3 by adapting readout step 24.More particularly, after having switched signals READ1 and READ2 totheir high values to cause a corresponding offset of the potential ofnodes 1451 and 1452, and thus the switching to the on state of outputtransistor 132, a first potential of node 136, corresponding to a firstoutput signal of the pixel, is read out and stored by the readoutcircuit connected to node 136. The first potential depends on thepotential of node 1451, and thus on the voltage across capacitor C1,which is representative of the potential of node 104 after reset step20. Transistor 1401 is then set to the on state, which causes amodification of the potential of node 1451, and thus of node 136. Asecond potential of node 136, corresponding to a second output signal ofthe pixel, is read out by the readout circuit. The second potential isrepresentative of the voltage across capacitors C1 and C2, and thus ofthe voltage across capacitor C2 at the end of step 23. The quantity oflight received by photodiode 100 during the previous integration phasecan then be deduced from the first and second output signals.

Similarly to pixel 1 described in relation with FIG. 1, pixel 1′described in relation with FIG. 3 enables to avoid the provision of arow selection transistor, between transistor 132 and node 136.

Pixel 1′ of FIG. 3 may be used to form an image sensor similar to thatdescribed for pixel 1, with the difference that, in each pixel column,output node 136 is common to all the column pixels and is coupled,preferably connected, to an input of the readout circuit. The operationof such a sensor is similar to that of the image sensor previouslydescribed in relation with pixel 1 of FIG. 1.

FIG. 4 shows an embodiment of a circuit of a pixel 2 according to asecond aspect of the present disclosure.

As compared with pixel 1 of FIG. 1, in pixel 2, transistor 122 has beeneliminated and node 120 is no longer a node of application of a powersupply potential but rather a node of application of an analog signal orpotential VSF for controlling transistor 116. Here, node 120 is(directly) connected to a conduction terminal of transistor 116, itsdrain. Thus, in the shown example, inner (intermediate) node 118 is onlyconnected to readout transistor 116 and to storage selection transistors1281 and 1282.

Further, in the specific example of FIG. 4, nodes 1381 and 1382 arenodes of application of a bias potential, for example, ground GND.Further, the output circuits of the pixel connected between node 118 andrespective nodes 1361 and 1362 comprise row selection MOS transistors,respectively 1501 and 1502. Transistor 1501 is connected betweentransistor 1321 and output node 1361. Transistor 1502 is connectedbetween transistor 1322 and output node 1362. The gates of transistors1501 and 1502 are connected to nodes, respectively 1511 and 1512, ofapplication of the respective digital control signals READ1 and READ2.In this embodiment, the high and low values of signals READ1 and READ2differ from those of FIGS. 1 to 3 and are, for example, respectivelypotential VDD and ground GND. Nodes 1511 and 1512 may be connected, withsignals READ1 and READ2 then corresponding to a same signal. In thisexample, transistors 1501 and 1502 are N-channel transistors, theirdrains being connected to the sources of respective transistors 1321 and1322.

FIG. 5 is a flowchart illustrating an embodiment of a method ofcontrolling pixel 2 of FIG. 4.

At a step 50 (Reset SN), sense node 104 is reset in the same way as atstep 20 of FIG. 2. Step 50 is implemented during an integration phase,preferably towards the end thereof.

During step 50, transistor 106 is maintained off and transistors 1281and 1282 are preferably maintained off. Control signals READ1 and READ2are preferably maintained at their low value so that transistors 1501and 1502 are off. Preferably, analog control signal VSF is maintained atits low value, for example, ground GND.

At a step 51 (Sample C1) following step 50, information representativeof the potential of node 104 is stored in capacitor C1. This stepcomprises two successive phases.

In a first phase, the value of signal VSF is progressively increasedfrom its low value, for example, ground GND, to its high value, forexample, potential VDD. As long as the potential difference betweennodes 104 and 118 is greater than the threshold voltage of transistor116, the potential of node 118 follows the potential of node 120. Thisresults in a decrease in the gate-source voltage of transistor 116. Whenthe gate-source voltage comes close to the threshold voltage oftransistor 116, the latter tends to turn off, the potential of node 118then tending to settle at a value approximately equal to that of thepotential of node 114 minus the threshold voltage of transistor 116. Thefirst phase ends when signal VSF reaches its high value and a secondphase starts.

The second phase is a delay phase of fixed duration, for example, 10 μs,during which signal VSF is maintained at its high value. Sincetransistor 116 is not totally off, the potential of node 118 slowlyincreases towards a value equal to the potential of node 104 minus thethreshold voltage of transistor 116. At the end of the second phase,signal S1 is switched to its low value, for example, the ground, to turnoff the corresponding storage selection transistor (1281 in thisembodiment). This enables to store the voltage across capacitor C1, andthus the potential of node 118, which is representative of the potentialof node 104. Signal VSF is then set back to its low value.

At a step 52 (Transfer) following step 51, the charges photogeneratedand stored in photodiode 100 during the integration phase aretransferred to node 104 in the same way as at step 22 described inrelation with FIG. 2. In particular, the switching of transistor 106from the on state to the off state implemented during step 52 marks theend of the current integration period and, for example, the beginning ofthe next integration period.

At a step 53 (Sample C2) following step 52, information representativeof the potential of node 104 is stored in capacitor C2.

Step 53 differs from step 51 in that, at the end of the second phase,signal S2 is switched to its low value, rather than signal S1, to turnoff the corresponding storage selection transistor (1282 in thisembodiment). This enables to store the voltage across capacitor C2, andthus the potential of node 118, which is representative of the potentialof node 104. The duration of the second phase of step 53 is identical tothat of step 51.

At a step 54 (READ C1, C2) of reading from the pixel, following step 53,output signals representative of the voltages stored across capacitorsC1 and C2 are read out. To achieve this, signals READ1 and READ2 areswitched to their high values to turn on transistors 1501 and 1502. Dueto the fact that transistors 1321 and 1322 are configured assource-followers and are biased by a readout circuit (not shown)connected to nodes 1361 and 1362, the potential of nodes 1361 and 1362depends on the potential of respective nodes 1371 and 1372. The outputsignals of the pixel are read out by the readout circuit from nodes 1361and 1362. The quantity of light received by photodiode 100 during theprevious integration phase can then be deduced from these outputsignals, particularly from the difference between these signals.

Steps 50 to 54 are repeated for each integration phase.

In pixel 2, the elimination of the transistor for biasing readouttransistor 116 is made possible by the fact that node 120 of pixel 2 isa node of application of a control signal rather than of a biaspotential. This enables to decrease the number of transistors of pixel 2with respect to a similar pixel which would comprise such a biastransistor.

FIG. 6 shows an alternative embodiment of the pixel circuit of FIG. 4.

Pixel 2′ of FIG. 6 differs from pixel 2 of FIG. 4 in that is comprises asingle output node 136 of the pixel and a single output circuitconnected between inner node 118 and node 136. This single outputcircuit is similar to that of pixel 1′ of FIG. 3.

More particularly, the output circuit of pixel 2′ differs from that ofpixel 1′ in that a row selection MOS transistor 150 is connected betweentransistor 132 and output node 136 and in that nodes 1441 and 1442 arenodes of application of a bias signal or potential, for example, groundGND, rather than of a control signal. The gate of transistor 150 isconnected to a node 152 of application of a row selection controldigital signal or potential, or output control signal, READ. In thisexample, transistor 150 has an N channel, its drain being connected tothe source of transistor 132.

The operation of pixel 2, described in relation with FIG. 5, applies topixel 2′ with the difference that the switching of signals READ1 andREAD2 to turn on or off transistors 1501 and 1502 are replaced withswitching of signal READ to respectively turn on or off transistor 150.Further, readout step 54 is adapted and comprises a switching of signalS1 to turn on transistor 1401, first and second output signals beingread from node 136, respectively before and after the switching oftransistor 1401 from the off state to the on state.

Pixel 2 of FIG. 4 or pixel 2′ of FIG. 6 may be used to form an imagesensor comprising an array of pixels 2 and 2′ organized in rows and incolumns, a control circuit or row decoder, and a readout circuit orcolumn decoder. The row decoder is preferably common to all the pixelsin the array. The row decoder supplies, simultaneously to all the pixelsof a same row, control signals TG, RST, S1, S2, VSF and, according tothe case, READ1 and READ2 (pixel 2) or READ (pixel 2′). The connectionof the output nodes of pixels 2 or 2′ to the readout circuit isidentical to what has been previously described in relation with FIGS. 1to 3. In such a sensor, for an operation of global shutter type, steps50, 51, 52, and 53 are implemented simultaneously for all the pixels ofthe array, and step 54 is implemented, row after row, simultaneously forall the pixels of a selected row.

FIG. 7 shows an embodiment of a pixel circuit according to a thirdaspect of the present disclosure.

Pixel 3 of FIG. 7 differs from pixel 1 of FIG. 1 in that:

-   -   nodes 114 and 124 are nodes of application of control potentials        or digital signals, respectively VRST and VBIAS, rather than        nodes of application of bias potentials, a conduction terminal        of transistor 110, here its drain, being (directly) connected to        node 114;    -   the output circuit, connected between nodes 118 and 1361, is        identical to that of pixel 2 of FIG. 4 and is connected between        the same nodes 118 and 1361, signal READ1 being for example        identical to that described in relation with FIGS. 4 and 5; and    -   the output circuit, connected between nodes 118 and 1362,        corresponds to that of pixel 2 of FIG. 4 and is connected        between these same nodes 118 and 1362, with the difference that,        in pixel 3, transistor 1282 has been eliminated, node 118 being        thus directly connected to the gate of output transistor 1322.        Signal READ2 is for example identical to that described in        relation with FIGS. 4 and 5.

FIG. 8 is a flowchart illustrating an embodiment of a method ofcontrolling pixel 3 of FIG. 7.

At a step 80 (Reset SN), implemented during an integration phase,preferably towards the end thereof, sense node 104 is reset. To achievethis, digital control signal RST is set to its high value, for example,potential VDD, to turn on transistor 110. Further, signal VRST is set toits high value, for example, potential VDD. Transistor 110 is thenturned off by the switching of signal RST to its low value, for example,ground GND. The potential of node 104 is then substantially equal to thehigh value of signal VRST. Signal VRST thus also corresponds to adigital control signal for resetting node 104.

During step 80, transistor 106 is maintained off and transistor 1281 ispreferably maintained off. Control signals READ1 and READ2 arepreferably maintained at their low value so that transistors 1501 and1502 are maintained off.

At a step 81 (Sample C1) following step 80, information representativeof the potential of node 104 is stored in capacitor C1. To achieve this,signal VBIAS is set to its high value, for example, 0.6 V, such that abias current flows through transistor 116. Thus, the potential of node118 depends on the potential of node 104 due to the fact that transistor116 is configured as a source follower. Signal S1 is then switched toits low value, for example, ground GND, causing the turning off ofstorage selection transistor 1281 and the storage of the voltage acrosscapacitor C1.

At a step 82 (Transfer) following step 81, the photogenerated chargesstored in photodiode 100 are transferred to node 104 by the switching ofsignal TG to turn on transistor 106. This results in a modification ofthe potential of node 104. Transistor 106 is then switched to the offstate, by the switching of signal TG. Such a switching of transistor 106from the on state to the off state marks the end of the integrationperiod and, for example, the beginning of the next integration period.During step 82, signal VBIAS is maintained at its high value, so thatthe potential of node 118, and thus the voltage across capacitor C2,depends on the potential of node 104.

At a step 83 (Sample C2) following step 82, information representativeof the potential of node 104 is stored in capacitor C2. To achieve this,transistor 122 is turned off by the switching of signal VBIAS to its lowvalue, for example, ground GND. As a result, transistor 116 is no longerbiased. Transistor 116 is then turned off by setting of signal VRST toits low value, for example, ground GND, capable of turning offtransistor 116 when it is applied to the gate of transistor 116.Transistor 110 is then turned on, by the switching of signal RST to itshigh value, to apply signal VRST to the gate of transistor 116. As aresult, the potential of node 118, and thus the voltage across capacitorC2, no longer varies. At the end of step 83, the potential of node 118,and thus the voltage across capacitor C2, is substantially the same asat the end of step 82 and is representative of the potential of node 104at the end of transfer step 82.

At a step 84 (READ C1, C2) following step 83, the voltages acrosscapacitors C1 and C2 are read out. Step 84 is identical to step 54described in relation with FIG. 5. During step 84, signals VRST, RST,and VBIAS are maintained at the same values as at the end of step 83.

Steps 80 to 84 are repeated for each integration phase.

The provision of a control signal on node 114, configured to switchtransistor 116 to the off state, enables to do away with the presence ofa storage selection transistor between node 1372 and node 118.

FIG. 9 shows an alternative embodiment of the pixel circuit of FIG. 7.

Pixel 3′ of FIG. 9 differs from pixel 3 of FIG. 7 in that it comprises asingle output node 136 of the pixel and a single output circuitconnected between inner node 118 and node 136. This output circuit issimilar to that of pixel 2′ of FIG. 6.

More particularly, the output circuit of pixel 3′ corresponds to that ofpixel 3′ where transistor 1422 has been eliminated. In other words, node118 is directly connected to transistor 1401, nodes 118 and 1452 beingconnected.

The operation of pixel 3, described in relation with FIG. 8, applies topixel 3′ of FIG. 9 with the difference that the switching of signalsREAD1 and READ2 to turn on or off transistors 1501 and 1502 are replacedwith switching of signal READ to respectively turn on or off transistor150. Further, readout step 84 is adapted and comprises a switching ofswitch S1 to turn on transistor 1401, the first and second outputsignals of the pixel being read from node 136, respectively before andafter the switching of transistor 1401 from the off state to the onstate.

Pixel 3 of FIG. 7 or pixel 3′ of FIG. 9 may be used to form an imagesensor comprising an array of pixels 3 and 3′ organized in rows and incolumns, a control circuit or row decoder, and a readout circuit orcolumn decoder. The row decoder is preferably common to all the pixelsin the array. The row decoder supplies, simultaneously to all the pixelsof a same row, control signals VBIAS, TG, RST, VRST, S1 and, accordingto the case, READ1 and READ2 (pixel 3) or READ (pixel 3′). Theconnection of the output nodes of pixels 3 or 3′ to the readout circuitis identical to what has been previously described in relation withFIGS. 1 to 3. In such a sensor, for an operation of global shutter type,steps 80, 81, 82, and 83 are implemented simultaneously for all thepixels of the array, and step 84 is implemented, row after row,simultaneously for all the pixels of a selected row.

FIG. 10 shows another embodiment of a pixel circuit 5. This embodimentcorresponds to the combination of the embodiments described in relationwith respective FIGS. 1, 4, and 7.

Pixel 5 of FIG. 10 differs from pixel 1 of FIG. 1 in that transistors122 and 1382 have been eliminated, and in that nodes 114 and 120 arenodes of application of the previously-described control signals,respectively VRST and VSF.

Thus, as compared with pixel 1, in pixel 5, node 118 is only connectedto transistor 1281 and to junction node 1372 of capacitor C2 and of thegate of transistor 1322, where nodes 1372 and 118 may be confounded.

An embodiment of a method of controlling pixel 5 comprises the followingsuccessive steps:

-   -   a first step where the potential of node 104 is reset, this        first step being similar to step 80 (FIG. 8, pixel 3);    -   a second step where information representative of the potential        of node 104 is stored in capacitor C1, this second step being        similar to step 51 (FIG. 5, pixel 2);    -   a third step where photogenerated charges stored in photodiode        100 are transferred to node 104, this third step being similar        to steps 22 (FIG. 2, pixel 1) and 52 (FIG. 5, pixel 2);    -   a fourth step where information representative of the potential        of node 104 is stored in capacitor C2. This fourth step        implements the two phases of previously-described step 53 (FIG.        5, pixel 2), with the difference that, at the end of the second        phase, transistor 116 is set to the off state by the switching        of signal VRST to its low value and then by the turning-on of        transistor 110, similarly to what has been described for step 83        (FIG. 8, pixel 3). Further, at the end of this fourth step,        signals READ1 and READ2 are switched to their low values to turn        off transistors 1321 and 1322; and    -   a fifth step where the output signals of pixel 5 are read out,        this fifth step being similar to step 24 (FIG. 2, pixel 1).

These steps are repeated for each integration period.

Pixel 5 may be used in a sensor of the type of those previouslydescribed. In particular, for an operation of global shutter type, thefirst, second, third, and fourth steps are simultaneously implementedfor all the pixels 5 of the sensor, the fifth step being implemented rowafter row, simultaneously for all the pixels 5 of the selected row.

FIG. 11 shows an alternative embodiment of pixel 5 of FIG. 10. Thispixel variation 5′ corresponds to the combination of the variationsdescribed in relation with respective FIGS. 3, 6, and 9.

Pixel 5′ of FIG. 11 differs from pixel 1 of FIG. 3 in that transistors122 and 1402 have been eliminated, and in that nodes 114 and 120 arenodes of application of the previously-described digital controlsignals, respectively, VRST and VSF.

Thus, as compared with pixel 1′ of FIG. 3, in pixel 5′, node 118 is onlyconnected to transistor 1401 and to capacitor C2, nodes 118 and 1452being confounded.

The control method described in relation with pixel 5 of FIG. 10transposes to pixel 5′ of FIG. 11 by adapting the fifth step of readingfrom the pixel so that it further comprises a switching of transistor1401 from the off state to the on state, the first and second outputsignals of the pixel being read from node 136, respectively before andafter the switching of transistor 1401.

Pixel 5′ of FIG. 11 may be used in an image sensor, in particular ofglobal shutter type, as described for pixel 5 of FIG. 10.

Although this has not been shown and detailed, the embodiments describedin relation with FIGS. 1, 4, and 7 may be combined two by two and,similarly, the alternative embodiments described in relation with FIGS.3, 6, and 9 may be combined two by two. It is within the abilities ofthose skilled in the art, in the light of the present disclosure, toimplement such combinations and to adapt the previously-describedcontrol methods to the pixels corresponding to these combinations.

In particular, in the case of a combination of the embodiments of FIGS.4 and 7, or of the alternative embodiments of FIGS. 6 and 9, during thestep of storage, in capacitor C2, of information representative of thepotential of node 104, the turning-off of transistor 116 at the end ofthe second phase of this step (delay period with signal VSF at its highvalue) is carried out by switching signal VRST to its low value and byswitching transistor 110 to the on state to turn off transistor 116.

FIG. 12 shows an embodiment of a circuit of a pixel 4 according to afourth aspect of the present disclosure.

Pixel 4 differs from pixel 1′ of FIG. 3 in that:

-   -   nodes 1441 and 1442 are nodes of application of a bias        potential, for example, ground GND;    -   transistor 132 has been eliminated;    -   node 124 is a node of application of a control signal BR taking        three constant values, that is, a low value, for example, ground        potential GND, a high value, for example, potential VDD, and an        intermediate value between potentials VDD and GND, where these        values may in practice be only approximately constant;    -   node 126 is selectively an output node of the pixel or a node of        application of a reference potential, for example, the ground;        and    -   a coupling capacitor Cc is (directly) connected between sense        node 104 and inner node 1451 having an electrode of capacitor C1        connected thereto, the other electrode of capacitor C1 being        connected to node 1441.

Capacitor Cc has a capacitance value smaller than those of capacitors C1and C2, for example, at least 10 times smaller, preferably at least 20times smaller. As an example, capacitors C1 and C2 have values ofapproximately 20 fF, for example, 20 fF, capacitor Cc having a value ofapproximately 1 fF, for example, 1 fF.

In the example of FIG. 12, a switch 1200 selectively coupling node 126of pixel 4 to a node 1201 of application of the reference potential orto a node 1202 of supply of an output signal of pixel 4 has been shown.

In the example of FIG. 12, the selection control signal is received byswitch 1200.

Such a pixel enables to use transistor 122 as a transistor for biasingtransistor 116 when signal BR is at its intermediate value, or as a rowselection transistor, or output transistor, when signal BR is at itshigh or low value. This enables to avoid the provision of a dedicatedrow selection transistor, and thus to decrease the number of transistorsin the pixel. Preferably, when transistor 122 is used as a transistorfor biasing transistor 116 or as a row selection transistor of pixel 4,node 126 is respectively coupled to node 1201 or to node 1202, by switch1200 in the example of FIG. 12.

FIG. 13 is a flowchart illustrating an embodiment of a method ofcontrolling the pixel of FIG. 12.

At a step 90 (Reset SN), the potential of node 104 is reset by theswitching of signal RST to its high value, for example, potential VDD,to turn on transistor 110. Transistor 110 is then turned off, by theswitching of signal RST to its low value, for example, ground GND.

Step 90 is implemented during an integration period, transistor 106 thenbeing off. Preferably, this step is implemented towards the end of theintegration period.

During step 90, transistors 1401 and 1402 are preferably on, signals S1and S2 being maintained at their high value, for example, potential VDD.Further, node 126 is coupled to node 1201 of application of thereference potential and signal BR is at its intermediate value, forexample, 0.6 V, so that a bias current flows through transistor 116.

Due to the fact that transistor 116 is configured as a source followerand is biased by transistor 122, the voltage across capacitors C1 and C2is representative of the potential of node 104.

At a step 91 (Sample C1) following step 90, while transistor 122 biasestransistor 116, the voltage across capacitors C1 and C2 is stored incapacitor C1. To achieve this, signal S1 is switched to its low value,for example, ground GND, to turn off transistor 1401.

At a step 92 (Transfer) following step 91, preferably carried out whiletransistor 122 biases transistor 116, the photogenerated charges storedin photodiode 100 are transferred to node 104 by the switching of signalTG to its high value, for example, potential VDD, to turn on transistor106. This results in a modification of the potential of node 104, andthus of the voltage across capacitor C2 due to the fact that transistor116 is configured as a source follower and is biased by transistor 122.It should be noted that such a variation of the potential of node 104has but a negligible influence on the voltage across capacitor C1 due tothe fact that capacitance Cc is small as compared with capacitance C1.Transistor 106 is then turned off, by the switching of signal TG to itslow value. Such a switching marks the end of the integration period and,for example, the beginning of the next integration period.

At a step 93 (Sample C2) following step 92, transistor 122 biasestransistor 116 and the voltage across capacitor C2, which isrepresentative of the potential of node 104, is stored. To achieve this,transistor 1402 is turned off by the switching of signal S2 to its lowvalue, for example, ground GND. At the end of step 93, signal BR is setto its low value to turn off transistor 122.

At a step 94 (Read C1, C2) following step 93, the pixel is read from,that is, two output signals representative of the voltages stored acrossrespective capacitors C1 and C2 are read from node 126 by a readoutcircuit coupled to this node, via switch 1200 in the example of FIG. 12.To achieve this, node 126 is coupled to node 1202 and transistor 122 isturned on by the switching of signal BR to its high value.

Due to the fact that transistors 106 and 110 are off and that capacitorCc couples node 1451 to node 104, the potential of node 104 is imposedby the potential of node 1451, and thus by the voltage across capacitorC1. Further, since transistor 1402 is off and transistor 116 isconfigured as a source follower, the potential of node 118 depends onthe potential of node 104, and thus on the voltage across C1. Thus, afirst signal representative of the voltage across C1 is read from node126.

Transistor 1401 is then turned on, by the switching of signal S1 to itshigh value. This results in a modification of the potential of node1451, and thus of node 126, which depends on the voltage which waspresent across capacitor C2 before the switching of transistor 1401.Thus, a second signal is read from node 126.

The quantity of light received by photodiode 100 during the integrationperiod can then be deduced from these first and second output signals ofthe pixel.

Preferably, at the end of step 94, transistor 122 is turned off by theswitching of signal BR to its low value.

Pixel 4 of FIG. 12 may be used to form an image sensor comprising anarray of pixels 4 organized in rows and in columns, a control circuit orrow decoder, and a readout circuit or column decoder. The row decoder ispreferably common to all the pixels in the array. The row decodersupplies, simultaneously to all the pixel of a same row, control signalsTG, RST, S1, S2, and BR. Preferably, in each pixel column, node 126 iscommon to all the pixels in the column. Preferably, common node 126 isselectively coupled to a node 1202 common to all the pixels in thecolumn or to a node 1201 common to all the pixels in the column, forexample, by a switch 1200 common to all the pixels in the column. As anexample, switch 1200 may form part of the column decoder, the node 126common to all the pixels of a column then corresponding to an input nodeof the column decoder, or may be external to the column decoder, node1202 then corresponding to an input node of the column decoder. In sucha sensor, for an operation of global shutter type, steps 90, 91, 92, and93 are implemented simultaneously for all the pixels in the array andstep 94 is implemented, row after row, simultaneously for all the pixelsof a selected row.

Various embodiments and variations have been described. It will readilyoccur to those skilled in the art that certain features of these variousembodiments and variations may be combined, and other variations willoccur to those skilled in the art. In particular, it is within theabilities of those skilled in the art to adapt the described embodimentsand variations to the case where the photogenerated charges which arestored in photodiode 100 are holes, particularly by replacing N-channeltransistors with P-channel transistors and by adapting the controlsignals of these transistors and the values of the reference, powersupply, and/or bias potentials, the power supply potential being forexample likely to be negative.

Finally, the practical implementation of the described embodiments andvariations is within the abilities of those skilled in the art based onthe functional indications given hereabove.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

The invention claimed is:
 1. A pixel, comprising: a photosensitivecircuit; a sense node; a first transistor; and a first capacitor havinga first electrode connected to a control terminal of the firsttransistor and having a second electrode connected to a node ofapplication of a first control signal; wherein said first control signalis controlled to have a negative voltage level following sampling of asignal at said sense node which produces a sampled signal on the firstcapacitor, and to switch to a non-negative voltage during a read-out bythe first transistor of the sampled signal.
 2. The pixel of claim 1,wherein the first transistor is connected between a node of applicationof a power supply potential and an output node of the pixel, the controlterminal of the first transistor being coupled to an inner node of thepixel.
 3. The pixel of claim 2, further comprising a second transistorhaving a control terminal connected to the sense node, having a firstconduction terminal connected to the inner node, and having a secondconduction terminal connected to a first node of application of one ofthe power supply potential or a second control signal.
 4. The pixel ofclaim 3, wherein the first node is a node of application of the secondcontrol signal and there is no transistor connected to the inner nodeand configured to deliver a bias current to the second transistor. 5.The pixel of claim 3, further comprising a third transistor having afirst conduction terminal connected to the sense node and having acontrol terminal connected to a node of application of a third controlsignal.
 6. The pixel of claim 5, wherein a second conduction terminal ofthe third transistor is connected to a node of application of the powersupply potential.
 7. The pixel of claim 5, wherein a second conductionterminal of the third transistor is connected to a node of applicationof a fourth control signal.
 8. The pixel of claim 5, wherein: the firsttransistor is an output transistor; the second transistor is a readouttransistor; and the third transistor is a reset transistor.
 9. The pixelof claim 7, wherein: the first control signal is an output controlsignal; the second control signal is a signal for controlling the secondtransistor; the third control signal is a reset control signal; and thefourth control signal is a second reset control signal.
 10. The pixel ofclaim 1, further comprising: a storage selection transistor connectedbetween the first electrode of the first capacitor and the inner node; asecond output transistor having a first conduction terminal connected toa node of application of the power supply potential, having a secondconduction terminal connected to a second output node, and having acontrol terminal coupled to the inner node; and a second capacitorhaving a first electrode connected to the control terminal of the secondoutput transistor and having a second electrode connected to a node ofapplication of a second output control signal.
 11. The pixel of claim10, wherein the first electrode of the second capacitor is connected tothe inner node or wherein the pixel comprises a second storage selectiontransistor connected between the first electrode of the second capacitorand the inner node.
 12. The pixel of claim 1, further comprising: asecond capacitor having a first electrode coupled to the inner node andhaving a second electrode connected to a node of application of a secondoutput control signal; and a storage selection transistor connectedbetween the first electrode of the first capacitor and a first electrodeof the second capacitor.
 13. The pixel of claim 1, further comprising atransfer transistor connected between the photosensitive circuit and thesense node.
 14. An image sensor comprising an array of pixels ofclaim
 1. 15. The pixel of claim 1, wherein controlling the first controlsignal to have the negative voltage forces the first transistor into anoff state and, wherein controlling the first control signal to have thenon-negative voltage permits turn on of the first transistor.
 16. Thepixel of claim 1, wherein the non-negative voltage is a ground voltage.17. A pixel, comprising: a photodiode; a sense node; a transfertransistor coupled between a node of the photodiode and the sense node;a first source-follower transistor having a control terminal coupled tothe sense node; a first selection transistor coupled between an innernode at a source terminal of the first source-follower transistor and afirst gate node; a first capacitor having a first electrode connected tothe first gate node and a second electrode coupled to receive a firstcontrol signal having a voltage that switches between a negative voltageand a non-negative voltage; and a second source-follower transistorhaving a control terminal coupled to the first gate node and a sourceterminal at a first output node.
 18. The pixel of claim 17, wherein thenon-negative voltage is a ground voltage.
 19. The pixel of claim 17,wherein the negative voltage at the second electrode of the firstcapacitor causes the second source-follower transistor to be in a turnedoff state, and wherein the non-negative voltage at the second electrodeof the first capacitor causes the second source-follower transistor tobe in a turned on state.
 20. The pixel of claim 17, further comprising:a second selection transistor coupled between the inner node at thesource terminal of the first source-follower transistor and a secondgate node; a second capacitor having a first electrode connected to thesecond gate node and a second electrode coupled to receive a secondcontrol signal having a voltage that switches between the negativevoltage and the non-negative voltage; and a third source-followertransistor having a control terminal coupled to the second gate node anda source terminal at a second output node.
 21. The pixel of claim 20,wherein the non-negative voltage is a ground voltage.
 22. The pixel ofclaim 20, wherein the negative voltage at the second electrodes of thefirst and second capacitors causes the second and third source-followertransistors, respectively, to be in a turned off state, and wherein thenon-negative voltage at the second electrodes of the first and secondcapacitors causes the second and third source-follower transistors,respectively, to be in a turned on state.
 23. A pixel, comprising: aphotodiode; a sense node; a transfer transistor coupled between a nodeof the photodiode and the sense node; a first source-follower transistorhaving a control terminal coupled to the sense node; a first selectiontransistor coupled between an inner node at a source terminal of thefirst source-follower transistor and a junction node; a first capacitorhaving a first electrode connected to the junction node and a secondelectrode coupled to receive a first control signal having a voltagethat switches between a negative voltage and a non-negative voltage; afirst selection transistor coupled between the junction node and a gatenode; a second capacitor having a first electrode connected to the gatenode and a second electrode coupled to receive a second control signalhaving a voltage that switches between the negative voltage and thenon-negative voltage; and a second source-follower transistor having acontrol terminal coupled to the gate node and a source terminal at anoutput node.
 24. The pixel of claim 23, wherein the non-negative voltageis a ground voltage.
 25. The pixel of claim 23, wherein the negativevoltage at the second electrode of the second capacitor causes thesecond source-follower transistor to be in a turned off state, andwherein the non-negative voltage at the second electrode of the secondcapacitor causes the second source-follower transistor to be in a turnedon state.